U.S. officials say there were 20,000 Russian-made anti-aircraft missiles in Libya before the uprising, and thousands have disappeared in the looting of Moammar Gadhafi's arm caches. According to the Washington Post, many of those Russian-made anti-aircraft weapons are being sold in Egyptian black markets, and so many are available the price has dropped from $10,000 to $4,000.

Egyptian officials told the paper they have intercepted looted Libyan weapons, including anti-aircraft guns, missiles and artillery, on the road from Libya into Egypt, in black markets on the Sinai Peninsula, and in the smuggling tunnels between the Sinai and Gaza.

The heat-seeking surface-to-air missiles, most of them shoulder-fired, have a range of two miles and would pose a threat to Israeli helicopter and planes on either side of the Israel-Gaza border.

Though Libya had an estimated 20,000 man-portable surface-to-air missiles before the popular uprising began in February, Assistant Secretary of State Andrew Shapiro told ABC News in September the government does not have a clear picture of how many missiles they're trying to track down.

A spokesperson told ABC News that the State Department "commend[s] Egyptian authorities" for seizing the missiles and other arms.

"[W]e are seeking additional information from Egyptian authorities as their investigations continue," said Noel Clay. "Egypt is one of several nations in the region where we have held discussions about potential conventional weapons proliferation from Libya in recent months. It is clear that the Egyptian government shares our concerns about weapons smuggling."

"Preventing the proliferation of these weapons is of international concern," said Clay. "We are committed to working with the Libyan Transitional National Council (TNC), Libya's neighbors, and the wider international community to build a coordinated approach to this shared security challenge."

U.S. government officials and security experts have long been concerned some of the thousands of heat-seeking missiles, along with smaller arms, could easily end up in the hands of al Qaeda or other terror groups.

"Matching up a terrorist with a shoulder-fired missile, that's our worst nightmare," Sen. Barbara Boxer, D.-California, a member of the Senate's Commerce, Energy and Transportation Committee, said in September.

The missiles, four to six-feet long and Russian-made, can weigh just 55 pounds with launcher. They lock on to the heat generated by the engines of aircraft, can be fired from a vehicle or from a combatant's shoulder, and are accurate and deadly at a range of more than two miles.

ABC News visited a massive weapons depot in Tripoli, and confirmed that the arms were unguarded and available to whoever could cart them away. Crates upon crates of weapons and ammunition, from AK-47 assault rifles to grenades to surface-to-air missiles, were forced open and their contents missing.

Peter Bouckaert, emergencies director of Human Rights Watch, told ABC News that "once these missiles go missing they are not that large and they can be easily hidden. It's very difficult to track them down. They can be moved across borders and can be very easily used.''

"Those are not weapons you want floating around this already volatile region," said Bouckaert.